Light-Switchable Polymers of Intrinsic Microporosity

Article abstract of DOI:10.1021/acs.chemmater.6b02619

The interest in (micro)porous systems is greater than ever before with microporous polymers finding application in areas such as gas storage/separation and catalysis. In contrast to the vast majority of publications on microporous polymers seeking ever higher values for surface area or uptake capacity for a particular gas, this work presents a means to render a microporous system responsive to electromagnetic stimuli. The incorporation of a diarylethene (DAE) derivative in the backbone of a polymer of intrinsic microporosity (PIM) produces a microporous system that exhibits photochromism as proven by UV-vis absorption and NMR studies. In the resulting DAE-PIM, surface area is not a fixed unalterable property but can be influenced by the external and nondestructive stimulus light in a reversible manner. Furthermore, in combination with Matrimid, free-standing membranes can be produced that display light-switchable diffusivity and permeability for carbon dioxide and oxygen. In this way, material scientists are offered the potential to employ only one system that can assume several states with different properties for each.

Full text of DOI:10.1021/acs.chemmater.6b02619

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Article
Light-Switchable Polymers of Intrinsic Microporosity
Daniel Becker, Nora Konnertz, Martin Böhning, Johannes Schmidt, and Arne Thomas
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02619 • Publication Date (Web): 28 Oct 2016
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Chemistry of Materials
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Light-Switchable Polymers of Intrinsic Microporosity
Daniel Becker,*,† Nora Konnertz,‡ Martin Böhning,‡ Johannes Schmidt,† Arne Thomas*,†
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†Technische Universität Berlin, Institute of Chemistry, Hardenbergstraße 40, 10623 Berlin, Germany
‡Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 87, 12205 Berlin, Germany
photochromism, diarylethenes, polymers of intrinsic microporosity, light-switchable porosity
ABSTRACT: The interest in (micro)porous systems is greater than ever before with microporous organic polymers
(MOPs) finding application in areas such as gas storage/separation and catalysis. In contrast to the vast majority of publi-
cations on MOPs seeking ever higher values for surface area or uptake capacity for a particular gas, this work presents a
means to render a microporous system responsive to electromagnetic stimuli. The incorporation of a diarylethene (DAE)
derivative in the backbone of a polymer of intrinsic microporosity (PIM) produces a microporous system that exhibits
photochromism as proven by UV-vis absorption and NMR studies. In resulting DAE-PIM surface area is not a fixed unal-
terable property but can be influenced by the external and non-destructive stimulus light in a reversible manner. Fur-
thermore, in combination with Matrimid free-standing membranes can be produced that display light-switchable diffu-
sivity and permeability for carbon dioxide and oxygen. In this way, material scientists is offered the potential to employ
only one system that can assume several states with different properties for each.
Introduction
This work aims at infusing a microporous organic pol-
ymer with the properties of DAEs. Owing to their high
surface areas, chemical stability, the ease of synthesis and
the large amount of functionalities that can be introduced
into the polymer backbone15 microporous organic poly-
mers have found application in catalysis,16-20 energy con-
version and storage,21-24 gas sorption25-28 and separation,29-
Molecular photochromic switches constitute an intri-
guing class of molecules. Several species of photochromic
compounds have been developed such as fulgides,1-4 azo-
benzenes,5,6 spiropyranes,7,8 and diarylethenes9,10 (DAEs).11
The interest in DAEs has gathered momentum ever since
their discovery in 1988 by Irie and co-workers who con-
ducted seminal work to elicit understanding of their
switching behaviour.12 As p-type chromophores DAEs are
known to be devoid of thermochromism, a circumstance
of paramount importance in the light of their anticipated
application in data storage devices.13 DAEs show reversi-
ble cyclisation and cycloreversion enabled by a central π-
electron sextet. After light-induced cyclisation, yielding
the closed state, the aromatic character of the thienyl
rings gives way to a more extended conjugation of π-
electrons. It could be shown that electronic conductivity
can be switched and is higher in closed than in open
state.14 The two states of DAEs – open and closed – usual-
ly show absorption spectra sufficiently different for each
to be addressed by an individual wavelength.
31
and also in organic photovoltaics.32-35 In comparison to
microporous polymer net- and frameworks, polymers of
intrinsic microporosity (PIMs) are exceptional due to
their one-dimensional structure as they consist of linear
polymer chains rather than extended three-dimensional
networks. In PIMs porosity is derived from interstitial
space/voids generated by the frustration of space-efficient
packing of polymer chains by virtue of building blocks
that possess an inherent kink in their structure hence the
term polymers of intrinsic microporosity.36,37 A great ben-
efit of PIMs is their solubility in common organic sol-
vents, which is why they can be processed into films or
membranes, an option generally non-existent for the
aforementioned other types of microporous networks
apart from very few instances in which this has been
achieved.38-40
Isomerisation in DAEs effects two decisive alterations
within the molecule. Firstly, electronic properties are
varied and, secondly, the molecule adopts a more rigid
structure upon cyclisation by interconnecting the thienyl
moieties and sacrificing mobility thereof. These two ob-
servations make it very intriguing to use DAEs in solid
compounds in order to attain a light-responsive material.
In case DAEs can be introduced into a porous system it
might be possible to vary e.g. gas uptake capacity, perme-
ability and selectivity by electromagnetic stimuli.
DAEs do perform a rather subtle geometrical change
during isomerisation as compared for instance to azoben-
zenes. Nevertheless, owing to DAEs in our system being
constituent parts of the very PIM-structure such a geo-
metrical change upon cyclisation and cyclreversion is
expected to exert a detectable influence on the entire
DAE-PIM chain more precisely on its ability to adopt a
more or less contorted conformation.
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In this work, functionalised, light-responsive DAE moi- with dropping funnel was charged with 3,5-dibromo-2-
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eties where used as one building block for the synthesis of
PIMs and the properties of resulting DAE-PIM during
light-triggered cyclisation and cycloreversion were stud-
ied in detail.
methylthiophene (1.0 eq., 30.24 mmol, 7.740 g) under Ar
flow. Dry diethyl ether (130 ml) was added. Mixture was
stirred and cooled to -78 °C to add n-BuLi (c = 1.6 mol·l-1
in n-hexane, 1.1 eq., 33.27 mmol, 2.131 g, 20.791 ml) drop-
wise via dropping funnel whereupon solution turned
yellow and warmed up. Mixture was then stirred for
90 min at -78 °C followed by slow addition of trimethylsi-
lyl chloride (2.5 eq., 75.60 mmol, 8.214 g, 9.663 ml) via
syringe. After a further 90 min of stirring at -78 °C cooling
was removed and solution allowed to cool to room tem-
perature. Reaction mixture poured into water (150 ml)
and layers were separated. Aqueous layer extracted with
diethyl ether (3×) and merged organic layers dried over
sodium sulfate, filtered off and evaporated to yield crude
product as yellow-brownish liquid (7.098 g, 94 %). Col-
umn chromatography (silica, n-hexane) yielded purified
product as colourless clear liquid (6.238 g, 83 %). 1H NMR
(200 MHz, CDCl3, δ): 7.03 (s, 1 H, Ar-H4), 2.43 (s, 3 H,
C2-CH3), 0.30 (s, 9 H, Si-(CH3)3); 13C NMR (100 MHz,
CDCl3, δ): 139.5 (C2), 137.9 (C4), 136.5 (C5), 110.8 (C3), 14.9
(CH3), -0.2 (Si-(CH3)3); HRMS (APCI) m/z: [M]- calcd for
C8H13BrSSi, 247.96851; found, 247.96840.
Experimental Section
Instrumentation and Materials: NMR spectra were rec-
orded either on a 400 MHz Bruker Avance II or a
200 MHz Bruker Avance II (software: Bruker Topspin 3.2).
UV-vis investigations were conducted using a Varian Cary
300 Scan UV-visible Spectrophotometer (software: Varian
Casy WinUV). Elemental analyses were conducted on a
Thermo FlashEA 1112 Organic Elemental Analyzer. Mass
spectrometry experiments were conducted either on a
LTQ Orbitrap XL (atmospheric-pressure chemical ionisa-
tion (APCI) and electrospray ionisation (ESI)) or a MAT
95S (electron impact (EI) ionisation). GPC was conducted
on a device from Thermo Separation Products in combi-
nation with a Shodex RI-71 detector. Illumination for the
sake of isomerisation was conducted either using a labor-
atory lamp usually employed for thin layer chromatog-
raphy (tlc) at 254 nm (6 W) for UV exposure or a Lumatec
Superlite S400 at 550 nm for exposure to visible light.
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Synthesis of 1,2-bis(2’-methyl-5’-trimethylsilylthien-3’-yl)-
hexafluorocyclopent-1-ene:14,41,42
A
flame-dried 500 ml
Bromine (99 %), 2-methylthiophene (98 %), n-
butyllithium, trimethylsilyl chloride (≥ 99 %), iodine
monochloride (100 %), bis(pinacolato)diboron (99 %),
2-neck Schlenk flask equipped with dropping funnel was
charged with dry THF (215 ml) followed by 3-bromo-2-
methyl-5-trimethylsilylthiophene (1.00 eq., 24.46 mmol,
6.096 g) via syringe. Solution was cooled to -78 °C to add
n-BuLi (c = 1.6 mol·l-1 in n-hexane, 1.10 eq., 26.903 mmol,
1.723 g, 16.815 ml) dropwise via dropping funnel. Mixture
was then stirred at -78 °C for 2 h whereupon octafluoro-
cyclopentene was added at -78 °C slowly via cannula as
follows: A separate flame-dried 25 ml Schlenk flask was
charged with dry THF (10 ml) and cooled to -78 °C. Oc-
tafluorocyclopentene (0.45 eq., 11.01 mmol, 2.334 g,
1.477 ml) was taken from the freezer and immediately
added to the separate flask via syringe (caution: com-
pound is extremely volatile, ϑboil = 27 °C). Dissolved oc-
tafluorocyclopentene was transferred to the prepared
reaction mixture via cannula and the small flask flushed
with dry THF (2×10 ml). Addition of octafluorocyclopen-
tene caused reaction mixture to assume a red-brownish
colour. Complemented solution was stirred at -78 °C for
4 h before cooling was removed and solution stirred for
another 18 h. Reaction was quenched by addition of hy-
drochloric acid (c = 0.1 mol·l-1, 100 ml) and mixture stirred
for 5 min whereupon the red organic layer was separated.
Aqueous layer was extracted once with diethyl ether,
which was merged with THF layer. Merged organic layers
dried over sodium sulfate, filtered off and evaporated to
obtain crude product as red liquid (6.609 g, 117 %), which
solidified in refrigerator. Column chromatography (silica,
n-hexane) in combination with recrystallisation
(n-hexane, fridge) of mixed fractions yielded clean prod-
uct as waxy, white solid (3.987 g, 71 %). 1H NMR
(200 MHz, CDCl3, δ): 7.05 (s, 2 H, H4), 1.90 (s, 6 H,
C2-CH3), 0.27 (s, 18 H, Si-(CH3)3); 13C NMR (100 MHz,
CDCl3, δ): 146.9, 138.9, 134.1, 133.5 (C6), 126.7 (C4), 121.4
(C7), 109.1 (C8), 14.2 (C2-CH3), -0.1 (Si-(CH3)3); HRMS
(APCI) m/z: [M]- calcd for C21H26F6S2Si2, 512.09132; found,
1,1’-bis(diphenylphosphino)ferrocene
1,1’-bis(diphenylphosphino)ferrocene
(97 %)
palladium(II)di-
(dppf),
chloride
(100 %),
tetrakis(triphenyl-phosphine)-
palladium(0) (99 %), potassium acetate (100 %) as well as
anhydrous (anh.) diethyl ether, anh. chloroform and anh.
1,4-dioxane were purchased from Sigma-Aldrich and used
as received. 2,2'-dibromo-9,9'-spirobi[9H-fluorene] (95 %)
was purchased from TCI and purified by column chroma-
tography (dichloromethane/n-pentane) prior to use. Oc-
tafluorocyclopentene was purchased from ChemPur and
used as received. Acetic acid (100 %) as well as solvents
other than the aforesaid were purchased from CarlRoth.
Synthesis
of
3,5-dibromo-2-methylthiophene:14,41,42
2-methylthiophene
(1.0 eq., 76.87 mmol, 7.546 g,
7.398 ml) was added to glacial acetic acid (154 ml). Solu-
tion stirred and dropwise addition of bromine (2.0 eq.,
153.74 mmol, 24.571 g, 7.920 ml) ensued whereupon solu-
tion turned orange-brown. After stirring at room temper-
ature for 5 h, an aqueous saturated solution of sodium
bicarbonate was carefully added to the reaction solution
(120 ml). Solution extracted with chloroform (5×). Organ-
ic layer washed with water (5×) and dried over sodium
sulfate, then filtered off and evaporated to yield crude
product as brown liquid (16.310 g, 83 %). Purification by
column chromatography (silica, n-hexane) yielded pure
1
product as pale yellow liquid (7.740 g, 39 %). H NMR
(400 MHz, CDCl3, δ): 6.85 (s, 1 H, H4), 2.34 (s, 3 H, CH3);
13C NMR (100 MHz, CDCl3, δ): 136.1 (C2), 132.0 (C4), 108.8
(C5), 108.6 (C3), 15.0 (CH3); HRMS (APCI) m/z: [M]- calcd
for C5H4Br2S, 253.83950; found, 253.83925.
Synthesis of 3-bromo-2-methyl-5-trimethylsilylthio-
phene:14,41,42 A flame-dried 3-neck 500 ml flask equipped
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Chemistry of Materials
512.09015. Anal. calcd for C21H26F6S2Si2: C 49.19, H 5.11,
S 12.51; found: C 49.06, H 5.35, S 11.97.
tetrakis(triphenylphosphine)palladium(0)
(0.05 eq.,
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0.10 mmol, 0.112 g). Reaction vessel degassed and back-
filled with argon thrice. A mixture of N,N-dimethyl-
formamide (14.885 ml) and THF (14.885 ml), degassed by
three freeze-thaw-cycles, was added to yield a suspension.
Aqueous potassium carbonate solution (c = 2 mol·l-1,
3.85 eq., 7.44 mmol, 1.029 g in 3.721 ml of water) was add-
ed, reaction vessel sealed by screw cap and solution again
degassed by three freeze-thaw cylcles. Solution heated to
120 °C under argon atmosphere with no reflux condenser
attached and stirred at that temperature for 42 h. Solvent
was evaporated to add a small amount of THF to dissolve
soluble compounds. Solution dropped into methanol
(300 ml) causing precipitation of a yellow-green powder
that was filtered off (frit, porosity 5, vacuum), then within
the frit redissolved in THF to dispose with any insoluble
residues. As-such obtained solution evaporated to dryness
then redissolved in THF (3 ml) and repricipitated from
methanol (150 ml). Precipitate filtered off (frit, porosity 5,
Synthesis of
2-bis(5'-iodo-2'-methylthien-3'-yl)hexa-
fluorocyclopent-1-ene (1):14,41,42 In a flame-dried 500 ml
Schlenk flask 1,2-bis(2'-methyl-5'-trimethyl-silylthien-3'-
yl)hexafluorocyclopent-1-ene (1.0 eq., 3.90 mmol, 2.000 g)
was dissolved in dry chloroform (153 ml). Solution was
cooled to 0 °C before iodine monochloride (4.0 eq.,
15.60 mmol, 2.533 g) was added in one portion under Ar
flow causing the solution to assume a red colour. Solution
was stirred at 0 °C for 4.25 h after which the cooling was
removed and the solvent evaporated. Obtained residue
was redissolved in chloroform (300 ml) and washed with
aqueous sodium thiosulfate solution (c = 0.5 mol·l-1, 2×).
Each aqueous layer was extracted with chloroform twice.
Merged organic layers were washed with water twice and
each resulting aqueous layer extracted with chloroform
once. Merged organic layers dried over sodium sulfate,
filtered off and evaporated to give crude product as red,
highly viscous oil (2.594 g, 107 %). Column chromatog-
raphy (silica, n-pentane/dichloromethane (DCM) 98/2
(V/V)) yielded clean product as pink, highly viscous sub-
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vacuum) to yield an yellow-green solid (1.033 g, 78 %). H
NMR (400 MHz, THF-d8, δ): 8.03-7.79 (m, 4 H, H6,6’,7,7’),
7.72-7.45 (m, 2 H), 7.45-7.28 (s, 2 H), 7.20 (s, 2 H, H4’’,4’’’),
7.14-6.98 (m, 2 H), 6.92 (s, 2 H, H1,1’), 6.71-6.57 (m, 2 H),
1.93-1.75 (m, 6 H, CH3); 13C NMR (101 MHz, THF-d8, δ):
150.3, 149.8, 143.2, 142.9, 142.1, 142.0, 138.2 (C2 with boronic ester),
134.0, 129.0, 128.8, 126.6, 125.9 (C1), 124.7, 123.2 (C4’’), 121.6,
121.5, 121.1, 35.1 (Cquat,pinacol), 30.7 (CH3pinacol), 14.5 (CH3DAE).
Anal. calcd for C40H22F6S2: C 70.58, H 3.26, S 9.42; found:
C 63.55, H 3.95, 7.55. GPC (polystyrene standard): Mn:
2305 g·mol-1, Mw: 7179 g·mol-1, PDI: 3.1, repeating units: 11.
1
stance (2.083 g, 86 %). H NMR (200 MHz, CDCl3, δ): 7.18
(s, 2 H, H4), 1.90 (s, 6 H, CH3); 13C NMR (100 MHz, CDCl3,
δ): 147.8 (C2), 136.2 (C3,6), 126.7 (C4), 70.9 (C5), 14.5 (CH3);
HRMS (EI) m/z: [M]- calcd for C15H8F6I2S2, 619.80555;
found, 619.80599. Anal. calcd for C15H8F6I2S2: C 29.05,
H 1.30, S 10.34; found: C 28.93, H 1.47, S 9.71.
Synthesis of
borolan-2-yl)-9,9'-spirobifluorene (2):43
250 ml Schlenk flask equipped with reflux condenser was
charged with 2,2'-dibromo-9,9'-spirobi[9H-fluorene]
2,2'-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
A
flame-dried
Results and Discussion
(1.0 eq., 4.19 mmol, 1.988 g), bis(pinacolato)diboron
(2.2 eq., 9.23 mmol, 2.343 g), 1,1’-bis(diphenylphosphino)-
ferrocene (0.1 eq., 0.42 mmol, 0.233 g), 1,1’-bis(diphenyl-
phosphino)ferrocene palladium(II)dichloride (0.1 eq.,
0.42 mmol, 0.307 g) and anhydrous potassium acetate
(6.0 eq., 25.16 mmol, 2.471 g). Solids were degassed in
vacuum with consecutive refilling with argon for four
times. Dry 1,4-dioxane (84 ml) was added via syringe and
mixture refluxed for 20 h (progress of reaction checked by
tlc). After cooling to room temperature, DCM and water
were added to separate organic layer. Aqueous layer was
extracted with DCM (3×) and merged organic layers
washed with water (2×), dried over sodium sulfate, fil-
tered off and evaporated to dryness to yield a black solid
as crude product. Purification by column chromatography
(silica, n-pentane/ethyl acetate 99/1 → 90/10 (V/V))
Synthesis and Characterisation
For the synthesis of DAE-PIMs diiodo-DAE derivative 1
was coupled with bis(boronic acid pinacol ester)spirobi-
fluorene (2) by Suzuki cross-coupling reaction (Figure 1).
Spirobifluorenes are rigid molecules exhibiting a kink in
their structure, the essential attribute to avoid space-
efficient packing of polymer chains and thus to obtain a
PIM. The starting materials were chosen such as to be
1
yielded clean product as white solid (1.310 g, 55 %). H
Figure 1. Synthesis of DAEꢀPIM via Suzuki crossꢀcoupling
NMR (200 MHz, CDCl3, δ): 7.91-7.80 (m, 6 H), 7.35 (dt,
J = 7.5, 0.9 Hz, 2 H), 7.17 (s, 2 H), 7.09 (dt, J = 7.6, 0.9 Hz,
2 H), 6.66 (d, J = 7.6 Hz, 2 H), 1.26 (s, 24 H, 8 x CH3); 13C
NMR (50 MHz, CDCl3, δ): 149.7, 147.7, 145.2, 141.6, 134.9,
130.6, 128.4, 127.7, 124.2, 120.6, 119.5, 83.8 (Cquat,pinacol), 66.07
(C9), 25.0 (CH3); HRMS (APCI) m/z: [M + H]+ calcd for
C37H38B2O4, 569.30290; found, 569.30310.
O
C
reaction. Superscripts and refer to open and closed isomer,
respectively. 3D show a detail of two repeating units. Hydrogen
and fluorine atoms are omitted for clarity.
applicable to Suzuki cross-coupling reaction in order to
synthesise an alternating AB-type co-polymer. The ob-
tained DAE-PIM is superbly soluble in THF and in open
state (DAEO-PIM) precipitates from methanol as yellow-
green powder. Conversely, when evaporated rather than
precipitated the compound forms an intensely yellow-
green and transparent film of brittle nature. Investiga-
Synthesis of DAE-PIM: A 250 ml Schlenk flask equipped
with reflux condenser was evacuated and refilled with
argon three times before being charged with 1 (1.0 eq.,
1.94 mmol, 1.200 g), 2 (1.0 eq., 1.94 mmol, 1.100 g) and
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tions
by
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Figure 2. Spectral evolution of UVꢀvis absorbance of DAEꢀPIM in dissolved state (dichloromethane, c = 4·10ꢀ5 mol·lꢀ1) during illuminaꢀ
tion with 254 nm (a), 550 nm (b), and alternating exposure to either UV or visible light for cycling experiments (c). Note that for the ring
opening process (b) visible light of much lower intensity was used than for the cycling experiment (c) yielding the difference in exposure
time of 2.86 h as compared to only 90 s. In the insets in a) and c) focus is set on the intensity of absorbance at 610 nm.
permeation chromatography (GPC) revealed a molecu-
lar weight of approximately 7200 g·mol-1, which corre-
sponds to a polymerisation degree of n ≈ 11. These rela-
tively low values are presumably responsible for the fragil-
ity of the films. The polydispersity index of 3.1 for
DAE-PIM is comparable to the value reported for PIM-1
and other PIMs.44
lines in Figure 2a and inset) as well as a deviation from
the isosbestic point at 358 nm is observed after irradiation
times of more than 90 s, hinting at decomposition and/or
side reaction induced by sustained UV irradiation. For
this reason, a time of 60 s was used for irradiation with
UV light in further UV-vis related experiments in solution
as absorbance at 610 nm reached its apex after this time
already. The discussed colour change from yellow-green
to blue upon irradiation with UV light could be reversed
under illumination with visible light applying a wave-
length of 550 nm. Figure 2b shows the decrease in intensi-
ty of the band at 610 nm with a parallel increase of the
bands at 285 and 337 nm thereby regenerating the state
prior to UV illumination. Cycling experiments showed
that the photo-induced switching between open and
closed state is reversible, even though after five cycles the
intensity of the band at 610 nm reached only 80 % of the
initial value (Figure 2c and inset).
According to nitrogen sorption experiments the film
shows essentially no porosity whereas the powder is rea-
sonably porous exhibiting a surface area according to the
theory of Brunauer, Emmett, and Teller (SABET) of
395 m2·g-1 (Figure S2). The lack of porosity in the film-like
morphology is evident in both open and closed form of
DAE-PIM.
Investigation of Photochromism
Successful introduction of photochromism was re-
vealed by UV-vis experiments which were conducted in
both solution and solid state. The obtained spectra from
investigation of the dissolved state and corresponding
photographs of the solution in different states of illumina-
tion are presented in Figure 2. The data clearly show a
change in absorbance upon irradiation at 254 nm. Initial-
ly, DAE units are open and conjugation of π-electrons is
mainly confined to thienyl moieties. As a result, nearly no
absorbance in the visible range is observed except for a
slight absorption event from 400 to 500 nm rendering the
bulk compound yellow-green. DAEO-PIM does absorb in
the UV range with a maximum at 337 nm accompanied by
a shoulder and a minor absorption at 285 nm. Upon irra-
diation at 254 nm the solution turns from light yellow-
green to blue. This change is seen in the UV-vis spectra
with the emergence of a new band at 610 nm and a con-
comitant decrease of the bands at 285 and 337 nm. The
appearance of strong absorbance in the visible range is
triggered by a narrowing of the molecule´s bandgap in-
duced by an augmented conjugation of π-electrons upon
cyclisation. Noteworthy, a photo-stationary-sate could
not be detected. Instead, a loss of absorbance (dashed
DAEs do not perform a large geometrical change during
either cyclisation or cycloreversion, a circumstance re-
sponsible for a beneficial peculiarity of DAEs namely their
capability to perform isomerisation in solid state.45
Whether or not this holds true also for DAE-PIM was
scrutinised by virtue of UV-vis transmission measure-
ments conducted on a drop-cast film. Indeed, the initially
yellow-green film turns violet upon UV irradiation and
reverts to its erstwhile appearance after illumination at
550 nm (Figure S3). Switching in the solid state was not
only observed in a drop-cast i.e. evaporated sample, but
also in the powders obtained via precipitation from meth-
anol (Figure S4). Notably, UV-measurements on DAE-
PIM in solid state indicate reversible light-triggered con-
version between the open and closed state. Moreover, it
can be observed that degradation of DAE moieties within
the PIM is at least retarded in the solid state as compared
to the polymer in solution. An investigation into the con-
version rate of DAE-PIM was judged unreliable as the
polymeric character of our system leads to NMR signals of
pronouncedly broad shape even in solution thus imped-
ing any such inquiry.
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The isomerisation events – cyclisation and cyclorever-
accumulation of micropores with defined diameters in
this region can be expected, this increase in uptake might
be an intrinsic feature of the molecular structure of the
compound possibly caused by a gate-pressure effect
which opens up the structure thereby enhancing nitrogen
uptake capacity.48,49
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sion – set in motion pronounced changes in the distribu-
tion of electrons hence in the electronic situation. That
change in chemical environment within the molecule can
be traced by NMR spectroscopy (Figure S5).
When DAEC-PIM was stored in the dark at room tem-
perature no reversion to the open isomer was observed as
evinced by the intense blue colour that was retained even
after weeks of storage. Such behaviour was also witnessed
in systems related to metal organic frameworks.46,47
The reasoning for the observed decrease in SABET after
cyclisation emanates from the nature of porosity preva-
lent in PIMs. In contrast to (micro)porous organic net-
works in which porosity is created within each individual
polymeric (macro)molecule by a spanning of pores by
highly crosslinked struts and nodes,[15,25] PIMs can exhibit
porosity only when several individual chains congregate
in a space-inefficient fashion. Therefore the stiff but
kinked structure of PIMs is of crucial importance for the
observed inefficient chain packing and consequently de-
termines the percolated free volume accessible to nitro-
gen, i.e. microporosity of the polymer in the dry state. It
has been noted that already subtle changes in the flexibil-
ity or degree of contortion of the backbone structure of
PIMs can have a large influence on the finally observed
porosity.50
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Influence of Isomerisation on Gas Uptake
After it was soundly established that the synthesised
DAE-PIM does undergo photochemical isomerisation, it
was of interest whether porous characteristics can be
influenced by light stimulus. To minimise any unsolicited
influence by externalities such as precipitation procedure
DAEO-PIM was switched in solid state as powder ob-
tained via precipitation from methanol. UV irradiation
clearly effects a change in colour of the material giving
proof of proceeding isomerisation. Nitrogen sorption of
the as-switched sample indeed bespeaks a distinctly al-
tered situation with a decrease in SABET as well as a differ-
ent shape of the isotherm. SABET drops from 395 to
168 m2·g-1 for DAEO-PIM and DAEC-PIM, respectively
(Figure 3). After illumination with visible light to cause
cycloreversion and regain the initial state the sample did
assume its former colour (cf. Figure S4) and SABET did
rise to 245 m2·g-1, less than the initially measured value for
the open state but substantially more than the figure
recorded for the closed state. It was thus shown that illu-
mination with UV light to induce cyclisation results in a
decrease in surface area that can, at least partially, be
reverted by cycloreversion triggered by irradiation with
visible light. That SABET after cycloreversion does not
reach the value of initially measured DAEO-PIM might be
ascribed to degradation of DAE moieties (cf. Figure 2a)
that already took place and forestalls more comprehen-
sive cycloreversion.
Even though DAEs do not perform a large geometrical
change during either cyclisation or cycloreversion, the
molecule still adopts a rigid and essentially planar struc-
ture in the closed state but a slightly bent geometry in the
open form. Multiplied by each repeating unit of the chain
a higher degree of contortion within a polymer chain can
be expected when all DEA moieties are existent in their
open form, which in turn should yield a less space-
efficient packing yielding more free volume/interstitial
space between the chains thus higher SABET values.
That this material is extremely sensitive to structural
changes is emphasized by the fact that after the powder
has been illuminated for 90 s at 254 nm the conversion
rate amounts to 5 % as compared to the maximal achiev-
able conversion as evidenced by UV-vis experiments
(Figure S6).
Influence of Isomerisation on Membrane Performance
Apart from surface area, it was examined whether gas
diffusivity and permeability can be switched by light
stimulus. Owing to its low molecular weight, DAE-PIM
does not form a free-standing membrane on its own.
However, as the compound is well soluble in various
common solvents it was possible to form a composite in
conjunction with Matrimid (MI), a commercially available
polyimide that is commonly used in gas separation mem-
branes (Figure S7). Membranes were prepared from a
chloroform solution containing both polymers yielding a
DAE-PIM@MI composite that consists of 5 wt% of
DAE-PIM.
Figure 3. Isotherms derived from nitrogen sorption at 77 K. Plotꢀ
ted are isotherms from DAEOꢀPIM (green, solid line), DAECꢀPIM
(purple line) and DAEOꢀPIM after reꢀopening (green, dashed line).
Switching was conducted in solid state (ss).
It was first necessary to verify whether or not DAE-PIM
performs cyclisation when incorporated in the Matrimid
matrix. Figure 4 shows UV-vis transmission spectra for
DAE-PIM@MI confirming that DAE-PIM does switch
inside the membrane, which assumes distinct colours for
the open and closed form in the process. The green curve
depicts the spectrum obtained for the composite as-
The obtained isotherm for the precipitated sample
shows an unusually sharp uptake of nitrogen in the region
below relative pressures of 0.1. As for amorphous PIMs no
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Page 6 of 9
blended.
bility of oxygen and carbon dioxide within the mem-
branes (Figure S9)
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Conclusion
In conclusion, it was shown that the light-responsive
properties of diarylethenes can be transferred to a poly-
mer of intrinsic microporosity by coupling of DAE and
spirobifluorene monomers. Resulting DAEO-PIM exhibits
reasonable porosity with a surface area approaching
400 m2·g-1. The polymer is soluble and isomerisation of
DAE moieties could be witnessed by means of UV-vis
transmission and NMR spectroscopy. Moreover, it was
proven that cyclisation and cycloreversion of the DAE
moieties occur in solid state, i.e. in films or powders of
DAE-PIM. For DAE-PIM powders surface area could be
altered upon isomerisation showing a decrease after cy-
clisation and an increase after cycloreversion. Further-
more, in combination with Matrimid a gas-permeable
membrane containing DAE-PIM as light-responsive com-
ponent can be formed and the properties of the mem-
brane most importantly the diffusion coefficient can be
switched by external light stimulus.
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Figure 4. UVꢀvis transmission spectra for DAEꢀPIM@MI superꢀ
imposed onto photographs of the composite in either open (left,
light green) or closed (right, dark green) state. Diffusion coeffiꢀ
cient for oxygen (circles) and carbon dioxide (squares) at 35 °C
and 10 bar of pure Matrimid (left) and DAEꢀPIM@MI (right) for
subsequent measurements in between which the composite was
switched by light.
It seems that it contains DAEC-PIM to a certain extent
evidenced by the absorbance at 600 nm. Initial illumina-
tion was performed using visible light (550 nm) to dimin-
ish the intensity of that band and reach a steady state of
DAEO-PIM@MI. Subsequently, the composite was irradi-
ated with UV (254 nm) and visible light in an alternating
fashion to conduct cycling experiments. The prolonged
illumination time with UV light has to be accounted for
by the applied, weak, light source (lamp for tlc, 6 W) as
well as the fact that Matrimid absorbs UV light to a con-
siderable extent thereby drastically reducing the amount
of light available to DAE-PIM inside the composite. The
inset in Figure 4 shows that although absorbance after
UV cycles is lower than for the first run it stays approxi-
mately the same after that.
However, it has to be stressed that for the anticipated
application the issue of fatigue resistance has to be ad-
dressed. The here applied DAE derivative is certainly not
optimized evidenced by the lack of any substituents that
could suppress fatigue such as methyl groups in 4 and 4’
position of the thienyl rings. Additionally, the employed
wavelength of 254 nm to trigger cyclisation is relatively
short and it is known that such short wavelengths can
lead to pronounced degradation for several reasons such
as reduced penetration depth and higher spatial concen-
tration of excited states and a higher variety of reaction
pathways that lead to by-products.53,54 For future work the
two issues of molecular design of the DAE moiety and the
utilization of longer wavelengths such as 313 nm or
365 nm will be addressed.
The concept presented in here is a promising approach
towards light-switchable membranes, in which diffusion
and permeability of gases can be controlled by an external
and non-destructive stimulus in a reversible manner.
Diffusion coefficients (D) for oxygen and carbon diox-
ide were measured for DAE-PIM@MI after alternating UV
and visible light illumination as well as for pure Matrimid
as reference. Generally, diffusion coefficients for oxygen
are higher than for carbon dioxide in pure Matrimid as
well as in DAE-PIM@MI membranes. This can be at-
tributed to the kinetic or effective diameter of the gas
molecules which is smaller for oxygen.51,52 Incorporation
of DAE-PIM into Matrimid leads to a slight increase in D
of approximately 10 % for both oxygen and carbon diox-
ide. With time and an increasing number of measure-
ments diffusion coefficients decrease in both pure Matri-
mid and the composite, a manifestation of physical ageing
occurring in both systems. Nonetheless, it is clearly seen
that isomerisation of DAE-PIM@MI by light stimuli has a
significant influence on the diffusion coefficient of the
membrane. Switching to the closed form lowers D where-
as cycloreversion results in an enhancement thereof. The
process can be repeated and a zigzag pattern of D can be
observed with DAEO-PIM@MI exhibiting higher values
than DAEC-PIM@MI.
As the switching process is accompanied by a conspic-
uous colour change systems can be envisioned that ex-
ploit the colour of the membrane to determine its current
state and adjust its gas separation performance (Fig-
ure S10).
ASSOCIATED CONTENT
Supporting Information.
Supporting information include results from nitrogen sorp-
tion of DAE-PIM in film-like morphology, UV-vis transmis-
sion spectra for films of DAE-PIM, photographs of isomerisa-
tion progress in DAE-PIM as powder, liquid-NMR spectra
recorded in various stages of the isomerisation process, theo-
retical and practical details for membrane formation and
time-lag measurements to determine diffusivity and permea-
bility as well as results for the latter. NMR spectra for the
compounds synthesised in this work are also included. This
A comparable trend is observed for the permea-
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http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Daniel Becker and Arne Thomas, E-mail:
daniel.becker@tu-berlin.de, arne.thomas@tu-berlin.de
ACKNOWLEDGMENT
9
We thank the European Research Council (ERC) (Grant:
278593ORGZEO) and the DFG (Cluster of Excellence UniCat
EXC 314/2) for financial support.
The authors would like to thank T. Rybak for support of film
casting and permeation experiments. Nora Konnertz is grate-
ful for support of the PhD-programme of BAM.
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ABBREVIATIONS
DAE, diarylethene; PIM, polymer of intrinsic microporosity;
MI, Matrimid; SABET, surface area according to the theory of
Brunauer, Emmett, and Teller; tlc, thin layer chromatog-
raphy; UV-vis, ultraviolet-visible; D, diffusion coefficient.
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